The invention concerns a device for measuring an electrical current that flows through a strip conductor, with a measuring conductor extending in the current flow direction, along and in the proximity of a measuring section of the strip conductor, one end of the measuring conductor being electrically connected direct with one end of the measuring section, the other end of the measuring section being provided with a first measuring point, and with a second measuring point being electrically connected with the other end of the measuring section.
In a known measuring device of this kind (DE 33 24 224 A1, FIG. 4), the strip conductor is bent to a U-shape, a measuring conductor being arranged on the outside of each leg of the U-shaped strip conductor, one end of each measuring conductor being electrically connected with one of the free leg ends. A strip conductor of this kind cannot be arranged on an insulating base plate, like the strip conductors of a printed circuit. Further, part of the current is branched off from the strip conductor via one of the measuring conductors and led to the other measuring conductor via a inverse feedback resistor of an operational amplifier, as well as voltage sources. The measured voltage is taken from the output of the operational amplifier, said measured voltage then being proportional to the inverse feedback resistance and the ratio of the resistance of the strip conductor as well as the sum of this resistance and the resistance of the measuring conductor. An operational amplifier can typically supply up to 10 mA. However, the current to be measured in the strip conductor can amount to 1000 A. Already with a current to be measured of 100 A, this results in a measuring conductor cross-section, which has a ratio of 10,000 to that of the strip conductor. In order to perform accurate measuring, this extreme cross-section ratio even has to be made with an accuracy of a few percent. This is practically not possible, when the strip conductor only has the usual width of a few millimetres. The cross-section of the measuring conductor would be too small and would not provide an accurately defined value, as required for an accurate measuring. Additionally, the inverse feedback resistor and additional voltage sources are expensive.
From EP 0 408 136 B1 is known an integrated circuit (IC), in which a current measuring occurs internally in the IC on a strip conductor, which has a known ohmic resistance over a known length. Via a slit, the strip conductor is divided into two partial currents, and at two measuring points on both sides of the strip conductor a voltage drop is taken as measure of the current.
The invention addresses the task of providing a measuring device as mentioned in the introduction, in which the sizes of strip conductor and measuring conductor can be selected more freely, particularly, a traditional strip conductor can be chosen, the cost of said measuring device being lower, even with a higher measuring accuracy, without causing measuring errors through inductive voltage drops at the measuring section.
This task is solved in that the measuring section is rectilinear, that the second measuring point is connected directly with the measuring section, that the measuring conductor is substantially current-less and that as measure of the current to be measured, a voltage is taken from the measuring points.
With this solution, the measuring is practically done current-less. A traditional, rectilinear strip conductor with the usual dimensions can be used. Also in the case of heavy current, the limits for the variation of the cross-section ratio of the strip conductor and the measuring conductor are wide. The costs are lower. Still, however, inductive voltage drops occurring in the measuring sections cause practically no measuring errors.
Preferably, the measuring conductor extends into the immediate proximity of the connecting spot of the measuring section and the second measuring point. Accordingly, the voltage, induced into the measuring conductor by a pulsating current flowing through the strip conductor, is higher, so that an inductive voltage drop in the measuring section is equalised very accurately.
According to the invention, the measuring conductor is arranged outside a plane defined by the strip conductor, the conductor being electronically connected to the strip conductor by an electrical connection, the device further comprising an insulating material which provides an electrical insulation between a major portion of the measuring conductor and the strip conductor.
In one embodiment, it may be ensured that the measuring conductor is arranged on an insulator fixed on the strip conductor. Via a heat conducting, electrically insulating layer, the strip conductor can, in a manner known per se, be arranged on a copper plate. With this solution, the copper plate and its heat conducting, but electrically insulating layer serves, on the one hand, as a substrate for the strip conductor and the measuring conductor as well as other electrical components and, on the other hand, also to transfer heat from the electrical components and the strip conductor. Here, eddy currents induced in the copper plate by an alternating current or a square wave current flowing in the strip conductor can also induce a voltage in the measuring conductor. As, however, in this case the measuring conductor is not in the same level as the strip conductor, but has a somewhat larger distance from the copper plate, which corresponds to the thickness of the insulator, the eddy currents merely induce a very low voltage in the measuring conductor, which hardly falsifies the measured value.
Preferably, the insulator is made of ceramic. This material has particularly good heat conducting and electrically insulating properties. At the same time, the ceramic and the material of the insulating layer applied on the copper plate can have substantially the same thermal expansion coefficient.
For fixing the insulator on the strip conductor, the bottom side of the insulator is preferably provided with a copper layer, which is brazed onto the strip conductor.
An alternative embodiment the measuring conductor could be surrounded by an insulating layer. This also gives a larger distance between the measuring conductor and a base plate of copper, on which the strip conductor is fixed by way of an electrically insulating layer.
An advantageous embodiment foresees that a temperature sensor is arranged near the strip conductor, said temperature sensor being connected with a control device of a power unit, which controls the power unit in dependence of a measured value of a load current, and communicating with a memory, that a correction factor, which corresponds to the deviation of the ohmic resistance of the measuring section from a predetermined nominal value, is stored in the memory, and that the control device multiplies each measured value of the load current flowing through the strip conductor by the correction factor. In this way, measuring errors are automatically equalised through a test measuring performed prior to the start-up by means of the known value of the current flowing through the strip conductor, in such a way that the correction factor is determined on the basis of the test measuring result and stored in the memory, from where it is downloaded by the control device during operation and used for the correction of the current measurings performed during operation.
The correction factor can compensate the dependence of the ohmic resistance of the measuring section on a manufacturing tolerance of the cross-section of the measuring section known in advance.
A second correction factor can compensate the dependence of the ohmic resistance of the measuring section on the temperature.
With a pulsating current having a ripple, the measuring value is preferably sampled during a pulse flank, preferably in its middle. Hereby, only the fundamental wave is measured and the current ripple is substantially eliminated from the measuring result.
In the following, the invention and its various embodiments are described in detail on the basis of the enclosed drawings, showing: